Today we call the original version as ISS1, and the modified version as ISS2.
What is the ISS3? This is the most enhanced version from 2005
![Smiley :)](/Smileys/phpbb/smiley.gif)
In 2005 I heard, someone in UK want to build Spectrum Emulator.
I drawed the schematics of the Emulator.
And when I drawing, I found other big hw bug!
The board have capabilty for turn OFF/ON the NMI generating via software. Using write bit 7 off port 44H.
But when turn off the NMI, after few seconds automaticaly turning on.. and hanging up the EXOS...
What is the problem?
The developers not know enought how works the Z80...
At the port decoding only use the -IORQ signal, and not use the -RD or -WR. But when the Z80 accepting IRQ, then setting the -IORQ to low! (And the -RD, -WR stay a high)
Because this, the 44H port randomly overwritten when accepting IRQs!
To correct this bug, I build additional logical gates to the board for combine the port selection signals with the -RD,-WR signals.
The plus ICs called as U17, U18.
After this modification, not needed to put switch to NMI line, the emulator software can turn on or off the emulator when needed.
I put one LED to a not used pin of U15. When the LED is lighting, the NMI generating is switched ON.
For this ISS3 version, I writed new version of ROM:
ZX version 4.1Other additions: load support for .TAP and .TZX files. .Z80 support will coming in the 4.2 version.
On the ISS3 version the exiting from Spectrum mode is different than ISS2. The reset button is make reset on the "Spectrum". If you want to exit, hold down the STOP key, and press reset.